The development of immature lungs is to be studied in three different systems. First, the lungs of young postnatal rats will be analysed histometrically in order to analyse the kinetics of alveolar development as a base line against which the effects of experimental interventions can be compared. These studies will be carried out on freshly removed lungs inflated at a constant air pressure so that they become preserved in a desiccated state. Thin slices from such lungs can then be mounted on slides and their alveoli measured and counted by standard histometric methods. Another investigation will involve the effects of maternal pneumonectomy on the development of fetal lungs. Right lungs will be removed from pregnant rats on the 16th or 19th days of gestation and the sizes of the lungs from their fetuses sacrificed at full term will be compared with those from sham operated mothers. These studies are designed to determine if humoral factors may exist which are responsible for promoting a vicarious response in the fetuses for their mothers' pulmonary deficiencies. Finally, unilateral pneumonectomies will be attempted in utero on sheep fetuses in order to determine if the prefunctional lung is capable of undergoing compensatory hypertrophy.